Frequently, researchers desire to analyze the ability of various proteins and other factors to bind to cell surfaces. Usually, the type of binding studied is one where the binding factor (ligand) recognizes and binds to a specific receptor for it on the cell surface. Thus, these types of studies are used to examine the inherent properties of the ligand itself, but also are used solely to study the receptor. Analysis of ligand binding to cell surfaces is usually performed directly, wherein that ligand itself is obtained in pure form and is radiolabeled. Usually, ligands are radiolabeled with 125I. More rarely, they are purchased labeled with 3H or 14C. The labeled ligand is assessed for its maintenance of activity, and for its specific (cpm per unit of weight) radioactivity. To measure binding, the radiolabeled material is applied under established optimal conditions to desired cells of known density (cells/unit volume or cell protein/unit volume). Typically, various concentrations (from high to low) of the ligand are added to separate tubes or dishes of cells. Certain cell containers at each dose tested also receive an excess of unlabeled pure ligand. Usually, these excesses are 10-200 fold times the concentration of labeled ligand. After the desired binding time has passed, the unbound material from all samples is saved and the cells are washed free of all unbound labeled and unlabeled ligand. The cells are then placed into counting tubes and counted for radioactivity. Initial unbound material is counted also. The amount of labeled ligand bound or unbound is calculated from the known specific cpm. Counts obtained from unlabeled excess ligand-receiving samples are subtracted from the counts obtained from samples treated with like-dose labeled ligand only. This provides specific cpm bound. The weight amount of specific labeled ligand bound is calculated from the known specific cpm per unit weight. Knowing the cell density, one can calculate amount of specific ligand bound per cell at each ligand dose level. Usually, the data is plotted as specific ligand bound/ligand unbound/unit of cells on the y axis and specific ligand bound/unit of cells on the x axis. This produces data with a negative slope and the x-intercept is the maximum amount of ligand able to bind. Therefore, the x-intercept also represents the receptors/cell for the ligand. This type of analysis is referred to as a Scatchard analysis. (Inoue et. al., 1993; LaGrange et. al., 1993; Schaffer, 1994; Gordon, 1995; Cavanaugh and Nicolson, 1998; Cavanaugh et. al., 1999).
An alternative method to determine ligand binding to cells is to conjugate a particular fluorescent molecule to the pure ligand. Fluorescent labeled material is allowed to bind to cells at various concentrations with or without the presence of unlabeled ligand. After binding is complete, all unbound ligand is washed off and the fluorescence of the cells is determined using a fluorescent spectrophotometer or a fluorescent activated cell sorter instrument (Gordon, 1995; Niedergang et. al., 2000; Palupi et. al., 2000, U.S. Pat. Nos. 5,576,436, 5,962,223, 6,027,890, 6,815,212). This procedure is more difficult to standardize and precise quantitation of ligand receptors/cell is not as accurate as with Scatchard analysis using radiolabeled ligand. This method is more given to comparing binding capacity between two different cell populations. With fluorescent activated cell sorting, it also requires that the binding surface exist in a monodispersed state capable of being analyzed in the flow cell of that instrument.
It is also possible to allow ligand binding to cell surfaces and to then incubate the cells with a fluorescent labeled antibody to the ligand, wash, and analyze cell fluorescence by fluorescent spectrophotometry or fluorescent activated cell sorting. To assess ligand receptor levels only, one can incubate cells with a fluorescent labeled antibody to the receptor and measure the fluorescence of the cells by fluorescent activated cell sorting (Cavanaugh and Nicolson, 1998; Cavanaugh et. al., 1999).
Western blotting is a technique where cell lysates obtained by detergent treatment are separated by electrophoresis and the separated components contained within the electrophoresis gel are driven onto a protein-binding membrane via electric current. The membrane with its cell constituents separated by molecular weight is blocked with a non-specific protein and can than be analyzed for particular cellular constituents by treatment with an antibody to that constituent followed by treatment with an enzyme conjugated antibody to the first antibody. Enzyme containing regions of the membrane are detected using color-producing or light-emitting substrates for that enzyme.
Dot-blotting or slot-blotting is where the cell lysate is applied directly to a binding-membrane without prior separation by electrophoresis. The membrane is blocked and treated as described in the previous paragraph to detect particular cell constituents. Unlike Western-blotting, the molecular weight of detected material is not ascertained.
We found that the binding of transferrin to tumor cell surfaces correlated with the aggressiveness of those cells; i.e.: the more metastatic tumor cells bound more transferrin than did poorly metastatic cells (Cavanaugh and Nicolson, 1991, Cavanaugh and Nicolson, 1998; Cavanaugh et. al., 1999). These studies required that we accurately assess the transferrin binding capability of cells in question. Initially, this was performed by examining the ability of the cells to bind 125I-transferrin and the ability of non-labeled transferrin to inhibit that. Dealing with radioactive iodine has many drawbacks including the inherent hazardous nature of the material, its short shelf life, and expensive waste disposal. In searching for novel methods for measuring transferrin binding using non-radioactive procedures, we came upon the discovery that fluorescein-labeled transferrin would stimulate the growth of cells in culture similarly to native transferrin. We also found that fluorescein-labeled transferrin could be internalized by cells and that this internalization could be competed for by an excess of un-labeled (or native) transferrin. The apparent retention of biological activity by fluorescein-labeled transferrin lent us to examine other technologies available to specifically detect the labeled protein. Many antibody suppliers now sell anti-fluorescein antibodies. These were initially developed to detect fluorescein-labeled oligonucleotides hybridized to sample RNA on Northern blots. These same antibodies can easily detect fluorescein-labeled proteins on Western blots (Samuel et. al.; 1988, Haselbeck, et al., 1990, Haselbeck and Hosel, 1992). We next assessed as to whether or not the combination of these reagents together would allow for the detection of fluorescein-labeled protein bound to cell surfaces. Cells were treated with fluorescein-labeled transferrin with and without an excess of native transferrin. After an appropriate incubation period, the cells were washed extensively and lysed with a detergent containing buffer. The lysate components were separated by electrophoresis and electroblotted onto a nitrocellulose membrane. The membrane was blocked with non-fat dry milk and incubated with a rabbit anti-fluorescein antibody. The membrane was washed and incubated with goat horse radish peroxidase-conjugated anti-rabbit IgG. The membrane was washed again and treated with a light emitting (enhanced luminescence) substrate for horse radish peroxidase. One band at ≈70,000 in molecular weight was seen in all lanes loaded with cells that initially were exposed to fluorescein-labeled transferrin only. In lanes loaded with cells that had also received an excess of native transferrin, a markedly reduced band, or no band at all was seen. This method allowed for the sensitive determination of transferrin binding to cells without the need for radioactively labeled transferrin. Furthermore, the molecular weight of the bound ligand was verified via the electrophoresis step.
The major difference in the method of this ligand binding method in comparison to those of the referenced patents and literature papers is the final detection method. In our case, the bound hapten-ligand is detected by immunological means after solubilization (or cell lysis) and immobilization onto a membrane. In the referenced cases, bound radio-labeled ligand is detected on solubilized cells by counting; or in the case of fluorescent-labeled ligands, by fluorescent detection of the label on intact cells by optical means such as cytometry.